When it comes to managing eMarketing and portal projects, I've always been conflicted about which tools to use for the job.
Traditional "Waterfall" projects involve up front planning using Work Breakdown Structures (WBS), Gantt charts, and involve resource leveling; whereas Agile web development practices use CRC cards, white boards, Wiki's, PostIt Notes, and are keen on lot's of direct customer-developer interaction.
I prefer to use both styles and recently discovered a way to strike a balance between traditional and agile project management using Google Spreadsheets as a project management tool.

Google Spreadsheets (let's call them "GS" for short) are basically a web version of Microsoft Excel built right into the Google platform. I can create spreadsheets using a web browser and send links to other people to view or edit the same spreadsheet.
My GS project template has 5 sheets named "Requirements", "Tasks", "Issues", "Metrics", and "Tests" that closely relate to Agile best practices and principles.
"Requirements" are customer generated "stories" of how the software should behave.
"Tasks" are the individual supporting tasks involved to deliver the requirements. I have a column for estimated hours and a calculated costs column based on a fixed hourly rate.
"Issues" are the inevitable bugs and clean-up items involved with implementing a project. The customer is encouraged to provide feedback on this sheet.
"Metrics" are used to log measurements, typically on a temporal scale such as 2 week iterations. Traditional measurements like "Resource utilization", "Estimate vs Actual" or Agile "Velocity" metrics may be collected.
"Tests" are like Requirements, except they are used to document quantifiable software outcomes, such as "If a customer is from the U.K., the purchase price should include a VAT tax of X%". Example product prices and expected results are defined in the sheet. I envision integrating FIT with this sheet for integrated testing.
To be truly agile, I need to be able to quickly re-prioritize requirements, tasks, and issues based on day-to-day project changes. To do this, each sheet has a Priority column that can be used for sorting rows up or down.
What makes this solution really rock is the ability to publish this spreadsheet online and share it with others. For example, I can easily publish a link to the Project Template you see in the screenshot above.
If I've shared the spreadsheet with a co-worker or customer and they login, I get a pop-up telling me they are online and we can use Google's web-based instant messenger to chat about the project.
Changes are reflected in near real-time and Google does a great job of warning you about potential conflicts if 2 people are updating the same cells.
What if I'm managing 15 concurrent projects and don't have the time to poll each spreadsheet for a status? No problem. GS has a built-in RSS feed that let's me subscribe to spreadsheet activity and let's me quickly see which projects have been updated.

Google's ubiquitous global presence allows our distributed project teams to stay in sync on a master record. This kind of dependence requires the occasional export to CSV or Excel XLS as a back-up measure.
This approach is not a silver-bullet for all the pains of project management, and it does have some down-sides.
Users have all or nothing permission to view or edit the spreadsheet. This means all the project costs (rolled up based on estimates) are visible to internal resources and Customers. This is not a problem in our environment because we have an open, fixed-bid project approach where the customers know what the cost is upfront and Developers are required to provide the estimates (and understand the project costs).
Another security issue is the fact that Google is still largely regarded as a consumer platform in the eyes of many, even though they've made great strides in delivering business quality services. Again, back it up frequently. There is no service level agreement.
You'll need a Gmail account to create spreadsheets and so will the people you're collaborating with. This is a free service. Google only requires you to help them with their viral marketing and identity management model.
No graphical charting. Not a requirement for agile anyway, but would be nice in the Metrics sheet for time-series analysis.
I took a template approach to creating the initial spreadsheet, but there is no "copy" or "clone" feature (at least that I can see). It takes some finagling to start a new project using a previously exported XLS template.
The "Labeling" concept takes some getting used to. The spreadsheets aren't stored in hierarchical folders. Google encourages you to create as many documents as you want and use customized Labels to tag or archive your docs.
Search does not appear to be supported in the current version (not that I was accustomed to it in Excel). But while you're working in Google it just seems natural to want to use search.
I suspect there will be some Salesforce integration in the near future, which will give me the ability to relate spreadsheets to Opportunities or Accounts. This would be a welcome addition to replace the gap created when the guys at iRows moved to Google.